It is known to provide an anti-eavesdropping window structure which comprises a plurality of glass panes including an outer pane and an inner pane separated from the outer pane by a spacer frame and which is filled with a gas and is provided with a sound generating unit to produce a sound which interferes with acoustic transmission through the window.
The assembly as thus described can be provided in a window frame and has been able, to a large measure, to prevent the pickup of speech and the like within the room provided that the window from the exterior utilizing directional microphones, vibration and acoustic pickup devices applied to outer parts of the window or to the outer pane, and even the most recently developed laser sensors which respond to vibration at the outer surface of the outer pane for eavesdropping purposes.
A disturbance-sound generator of the type which has been used in the prior art systems is here defined as a sound generator which is capable of delivering at a vibratile element, sound vibrations which are capable of setting the gas within the window structure in vibration and thus can transmit vibration to a structure to which this element is affixed or can radiate vibration into the gas-filled place.
Such disturbance-sound generators are usually electromagnetically energized by an electric current or with pulses and to this extent at least, the generator is an electroacoustic converter or transducer. The electromagnetic current or pulse train usually is generated in an electric-signal generator provided for this purpose and can be referred to as an interference-frequency generator.
A window structure of the aforedescribed type is described in German open application DE-OS No. 34 17 971, and, perhaps the best way of considering the system described in this open application is to consider points which will ultimately be found to distinguish it from the present invention.
Firstly, the structure itself is not treated as a sound-barrier structure and thus can be considered solely as an interference unit interposed between the sound to be protected and the potential eavesdropper. Indeed, the inner pane of this structure is a thick glass pane which is essentially the same thickness as an intermediate glass pane spaced therefrom by a relatively considerable distance toward the exterior and which are followed over a shorter spacing by a thin glass frame.
In the space between the two thick glass panes there is disposed an inclined glass pane which can, based upon its inclination, reflect sound waves. The compartments between the glass panes is filled with air and the interference-sound generator is a loudspeaker which generates the interference sound in the air filling of the inter-pane space.
There is no special indication as to what the frequency of the interference sound is and certainly there is nothing disclosed here as to any relationship of the structure to a sound attenuation effect with respect to transmitted sound.
With respect to general considerations, it is found that the described structure is expensive and for effective blockage of eavesdropping from the exterior, extremely high levels of interference sound must be generated in the window structure and this, of course, requires operation at very high energies.
The interference sound tends to pass into the room which is to be protected as well and creates a disturbance therein. There is no significant capability in this system to prevent laser-remote pickup from the outer surface of the window structure.
Thus, while using an interference-sound generator, the prior art window structure has been singularly unsuccessful in many respects to the point that its use has been avoided in many cases.